Apparently, I was under the (wrong) impression (up until recently) that as long as I did exercise in intervals of higher and lower intensity... that THAT was HIIT. Then I did some reading and found out that the high intensity intervals have to be pretty much the hardest your body can work, and that you shouldn't be able to last much longer than 30 seconds at that intensity.

So I have started sprinting as fast as I can during those intervals. When I get to the end of them, my legs feel like jelly, my heart is beating like crazy, and I feel nauseous. All of this goes away within the next 30 seconds, however....